Dar Ben,
Thank you for you immediate reply!
Last night I solved it.
The Apple Air Port Extreme routers may cause PPTP related troubles for “Linux users”. I found reports on this issue on the internet, dating from 2006 (maybe even earlier). My old mid priced router allowed this traffic.
The APE accepts PPTP from my Mac, Ipad and Adroid but not form the Raspberry Pi, most confusing.
Best regards
Johan